This application claims Paris Convention priority of DE 100 33 410.5 the entire disclosure of which is hereby incorporated by reference.
The invention concerns a cryostat arrangement for storing liquid helium, which consists of an outer shell, a helium container installed therein, and a neck pipe extending perpendicularly or at an inclined angle from the helium container to the outer shell whose upper warm end is connected to the outer shell and whose lower cold end is connected to the helium container, wherein the outer shell, the helium container and the neck pipe define an evacuated space containing a radiation shield surrounding the helium container and being connected at a coupling to the neck pipe in a heat-conducting fashion, wherein a refrigerator having a cold finger comprising at least one pipe and projecting into the existing neck pipe is installed into the neck pipe.
Conventional cryostat arrangements of this type (see e.g. U.S. Pat. No. 5,646,532) accommodate superconducting magnets used e.g. as main field magnets in magnetic resonance apparatus.
Superconducting magnets consist of windings of superconducting wire which are cooled down with liquid helium to temperatures of approximately 4.2 Kelvin. The main function of the cryostat arrangement is to keep the superconducting magnet at the predetermined operating temperature by means of liquid helium while thereby consuming as little liquid helium as possible.
The most important structural elements of cryostat arrangements are a helium container accommodating the superconducting magnet and liquid helium, one or more radiation shield(s) surrounding the helium container, an outer vacuum container (referred to below as the outer shell) and one or more neck pipe(s) connecting the helium container to the outer shell.
The helium container is surrounded by a vacuum space which is defined by the helium container itself, the neck pipes and the outer shell. The vacuum space reduces heat input into the helium container through convection as well as heat conduction through residual gas. Radiation shields are located between the helium container and the outer shell to reduce heat input via radiation. To charge the magnet with current, fill it with helium, and to permit evaporation of helium, one or more neck pipes are required which connect the helium container to the outer shell. The free cross-section of the neck pipes must be designed such that even the large amounts of helium gas, occurring e.g. during a so-called quench of the superconducting magnet, can flow off. In such a quench, the superconducting magnet spontaneously heats up to temperatures far above the boiling temperature of helium which causes conversion of liquid helium into helium gas which must flow through the neck pipe into the external space without causing an inadmissibly high pressure increase in the helium container. Such neck pipes can e.g. be made from stainless steel, titanium alloys or GFK. To keep the height of a cryostat low, the neck pipes disposed in the upper region of the cryostat usually have a length of approximately 1 m or less. They represent a heat bridge between the outer shell and the helium container. Neck pipes normally extend perpendicularly or slightly inclined from their lower cold end connected to the helium container towards their upper warm end connected to the outer shell.
The heat input into the helium container resulting from residual radiation, heat conduction through the neck pipes, and additional suspension members results in evaporation of the helium. Expensive helium must therefore be refilled at regular intervals. Since the evaporating helium cools the neck pipes and radiation shields coupled thereto, the heat input into the helium container is considerably reduced. The evaporation rate of liquid helium in cryostat arrangements for magnetic resonance apparatus without the active cooling described below is on the order of 0.1 l/h (liter per second) liquid or more.
To reduce costs associated with the refilling of expensive liquid helium, refrigerators are used in larger systems to provide active cooling. Such refrigerators are known e.g. from EP 0773450. They consist of a cold head mounted to a cryostat and its components, a compressor disposed at a separation from the cryostat, and pressure lines connecting the compressor to the cold head.
Cold heads for the applications mentioned herein usually have a mounting plate at room temperature, a cold finger mounted thereto and further components. During active cooling of cryostats, the mounting plate is almost always attached to the outer shell of the cryostat such that the cold finger projects either into a neck pipe or into a separate passage into the vacuum space. During operation, the end of the cold finger facing away from the mounting plate is cooled down to very low temperatures, e.g. 2-3 K.
The cold finger can consist of several pipes disposed parallel to one another which have different functions for generating an optimum cooling performance. Cold heads can have several stages. Thereby, a first stage disposed closer to the mounting plate is cooled to a first low temperature during operation, while the further stages are cooled to even lower temperatures.
The different stages of a cold head can be connected to the radiation shields and the helium container in a fashion which conducts heat well, to actively cool these components. Refrigerators for these applications can function e.g. according to the Gifford-McMahon principle or be designed as pulse tube coolers. Pulse tube coolers do not have any cold moving parts nor cold sealings which offers the advantage of long maintenance intervals and little mechanical vibration, which is advantageous for cryostats cooling magnets for magnetic resonance apparatus. In pulse tube coolers, the cold finger usually consists of two tubes per stage, disposed parallel to one another one of which is called the regeneration tube and the other the pulse tube.
Already in 1997, Thummes, Wang and Heiden have described in the document C. WANG, G. THUMMES, C. HEIDEN, CRYOGENICS 37,159 (1997) a refrigerator having a two-stage pulse tube cooler whose second stage produces a cooling performance of 170 mW at the boiling temperature of liquid helium of 4.2 K. This theoretically permits re-liquefying of evaporated helium gas at a temperature of 4.2 with a rate of 0.23 l/h of liquid.
In the document C. WANG, G. THUMMES, C. HEIDEN, CRYOGENICS 37,337 (1998) the same authors describe the use of this pulse tube cooler in a particular arrangement which permits cooling down of helium gas which was originally at room temperature and to liquefy same at a rate of 0.127 l/h of liquid. This is achieved in that the helium is guided through a thin tube which is wound about the regenerator pipes and is soldered thereto. Although the liquefying performance of the pulse tube cooler at 4.2 K is thereby reduced, the overall performance is considerably increased, since heat transfer is carried out at a higher than average temperature to thereby improve thermal efficiency.
Unfortunately, the cooling performance of these cold heads is too small to achieve either negligible or extremely small helium consumption in large magnet systems e.g. for magnetic resonance apparatus having high power input.
This is because in a cryostat arrangement that does not consume helium, the helium gas which normally results from evaporation and which flows off through the neck pipes to cool same is no longer available and the cold head must not only produce the power which the helium absorbs during evaporation, but also the power which the helium normally absorbs from the neck pipe through heating when rising therein. This heat power is many times the evaporation power which is required only for conversion of liquid into gas at the boiling temperature of 4.2 K.
In contrast thereto, it is the object of the present invention to improve a cryostat arrangement of the above described type with active refrigeration cooling through improvement of the thermal properties of the refrigerators. In particular, the invention should completely stop consumption of liquid helium.
This object is achieved in a surprisingly simple but effective fashion in that at least one pipe of the cold head installed into the neck pipe of the cryostat is surrounded by at least one separating body which divides the neck pipe into two partial volumes which are connected to one another both through a lower opening as well as an upper opening.
One partial volume directly borders the neck pipe and the other directly borders the above mentioned at least one pipe of the cold finger. The heat input through the neck pipe heats the helium in the partial volume bordering the neck pipe outside of the separating body while the helium in the partial volume within the separating body is cooled through the cooling power of the refrigerator.
This produces a temperature difference between the helium within the separating body and the helium between the separating body and neck pipe which again results in a different density of the two amounts of gas in the two partial volumes. The colder heavier amount of gas on the inside of the separating body flows downwardly and displaces, at the lower opening, the warmer and lighter gas between the neck pipe and the separating body thereby generating a convection cycle. The helium absorbs heat from the neck pipe during upward flow as in a conventional cryostat arrangement with helium consumption and, during flow, gives off heat to the cold head thereby pre-cooling the flowing-off helium.
This pre-cooling can be so efficient that further liquefying of the pre-cooled helium gas with high liquefying rates is possible, as mentioned e.g. in the publication C. WANG, G. THUMMES, C. HEIDEN, CRYOGENICS 37, 337 (1998).
In a preferred embodiment, the refrigerator can liquefy helium and condense the pre-cooled gas at the lower end of the separating body to completely stop consumption of liquid helium.
In a further preferred embodiment, the refrigerator is a pulse tube cooler. Pulse tube coolers are particularly suitable to pre-cool and liquefy originally warm gas due to their construction (see e.g. C. WANG, G. THUMMES, C. HEIDEN, CRYOGENICS 37, 337 (1998)). Moreover, pulse tube coolers are advantageous for cooling magnet systems in magnetic resonance apparatus due to their long maintenance intervals and low mechanical vibrations, which are considerably reduced with respect to all other refrigerator types.
In a preferred embodiment, the refrigerator has several stages. In this case, the first stage can be used e.g. for direct cooling of a radiation shield. It is moreover possible to produce particularly low temperatures with the last stage.
In a preferred embodiment, the separating body has poor heat conductivity to produce a large temperature difference between the gas outside and inside of the separating body. The drive of the helium cycle is thereby increased. The heat permeability xcex of the separating body should thereby be smaller than 10 kW mxe2x88x922Kxe2x88x921, preferably on the order of 100 W mxe2x88x922Kxe2x88x921 or less.
In a preferred embodiment, a material with good heat conducting properties connects one stage of the refrigerator to a radiation shield to permit direct active cooling of the radiation shield.
In an advantageous further development, the good heat-conducting connection between one stage of the cold head and a radiation shield is formed as a duct through the separating body. This permits direct cooling of a radiation shield by the stage of the refrigerator and also elongation of the separating body and thereby the spatial extent of the helium cycle from the lower end of the cold finger to its upper end, which can be favorable for the efficiency of the arrangement.
In a preferred embodiment, the lower opening of the separating body is at approximately the same height or below the lower end of the cold finger. In this fashion, the lower particularly cold regions of the cold finger are also utilized for cooling and liquefying the helium in the cycle.
An advantageous embodiment is characterized in that the lower end of the separating body is immersed in the liquid helium. Since the phase border surface within the separating body is too small to evaporate a sufficient amount of helium, condensation alone produces an underpressure in the inner partial volume of the separating body causing helium gas to flow out of the outer partial volume to the upper opening of the separating body. The helium cycle drive is thereby increased. Moreover, only an amount of gas flows which is equal to the amount of condensed gas thereby preventing excessively high convection.
In a preferred embodiment, the upper opening of the separating body is located below the first stage of the cold head thereby cooling only the region of the neck pipe in the helium cycle via the upwardly flowing helium. This region is directly connected to the helium container and the cooling thereof is particularly important for achieving negligible helium consumption. The construction of the separating body can also be simplified in this case.
In an alternatively preferred embodiment, the upper opening of the separating body is above the first stage of the refrigerator. In this fashion, all stages of the cold head are incorporated into the helium cycle. The upper opening in the separating body can also be designed as pipe-shaped connection between the two partial volumes and be guided out of the neck pipe.
Finally, in a further preferred embodiment, the separating body consists of an evacuated container completely surrounding at least one pipe of the cold finger, and at least one cooling pipe which is installed therein and is open at its ends and which is guided at an upper and a lower position in a vacuum-tight fashion through the evacuated container and, within the evacuated container, is in heat-conducting contact with at least one pipe of the cold head surrounded by the evacuated container. In this embodiment of the separating body, the inner space of the at least one cooling pipe forms one partial volume and the region surrounding the evacuated container forms the second partial volume and the ends of the at least one cooling pipe form the above mentioned openings in the separating body. In this fashion, it is in principle possible to optimize the efficiency of pre-cooling and subsequent liquefying of the flowing-off helium gas in the cooling pipe through design of the cooling pipe and via the detailed fashion by which the cooling pipe is guided along the different pipes of the cold finger.
Further advantages can be extracted from the drawing and the description. The features mentioned above and below can be utilized in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and further explained by means of embodiments.